Water companies in England and Wales have averaged five serious sewage spills into rivers or seas every day over the past decade, the Observer can reveal.

Analysis of Environment Agency data has found that the 10 firms recorded 19,484 category 1-3 pollution incidents between 2013 and 2022, the most recent year recorded, an average of one every four and a half hours.

Campaigners accused the water industry of “polluting our rivers and seas at a catastrophic scale”, while Labour said the government had “folded their arms and looked the other way” as the crisis worsened.

Thames Water was the worst offending company according to the Environmental Performance Assessments analysed by this newspaper, recording some 3,568 incidents in that time, followed by Southern Water (2,747), Severn Trent (2,712) and Anglian Water (2,572).

Most of the incidents recorded were category 3, the least severe of those collected and a type that is supposed to have only a localised effect.

But the figures are also likely to be an underestimate. The number and severity of sewage spill incidents are self-reported by water companies themselves.

The incidents, and their actual severity and impact, often go unverified. The Environment Agency, which regulates the sector, has faced staff shortages and major budget cuts that have forced it in the past to tell its inspectors to not investigate less serious incidents in order to cut down on costs.

Between 2020 and 2022, there were reportedly 931 pollution incidents in the north-west of England, but the EA attended just six.

A BBC investigation last year into the area’s water company, United Utilities, found that it had wrongly downgraded the severity of a significant number of its own sewage spills to lower categories, with the result that it avoided potential further scrutiny from the EA.

The state of the UK’s rivers and seas has become a major campaign issue in the general election.

Labour’s shadow environment secretary, Steve Reed, told the Observer that the Conservative government had “just folded their arms and looked the other way while water companies pumped a tidal wave of raw sewage into our rivers, lakes and seas”.

He said that Labour, if elected, would give regulators the power to ban bonus payments and even levy criminal charges for “law-breaking water bosses”.

The Liberal Democrats’ environment spokesperson, Tim Farron, whose rural Westmorland and Lonsdale constituency is one of the 25 worst affected by sewage releases, said the issue was a “national scandal which has gotten worse and worse under the Conservatives’ watch”.

“The Conservatives’ record is one of rising sewage levels and water firms stuffing their pockets with cash,” he added. “The Liberal Democrats have led the campaign against sewage, with plans for a new water regulator, an end to disgraceful bonuses and profits, and new sewage inspectors.”

In March it was revealed that England’s privatised water firms released raw sewage for a total of 3.6m hours in 2023, more than double the scale from the year before.

A small number of untreated sewage releases are allowed during periods of unprecedented rainfall when sewerage systems are overwhelmed, but a recent BBC investigation found thousands of “illegal” releases on days when it hadn’t rained, including during a record heatwave.

Despite the growing scrutiny of the industry in recent years, the pay packets for its leadership have remained high, with the nine chief executives of UK water companies receiving more than £25m in bonuses and incentives since the last general election. That has included bonuses awarded for hitting environmental and sustainability targets.

“This is further evidence of what we’ve all long suspected: water companies are polluting our rivers and seas at a catastrophic scale, each and every day,” said Giles Bristow, the chief executive of Surfers Against Sewage.

“These companies are brazen in their lack of regard for the law and have been allowed to pollute with barely more than a slap on the wrist.”

A Conservative spokesperson said the government had been clear that “water companies need to be held to account” and that they had “introduced unprecedented levels of transparency with 100% monitoring, and applied the largest ever fines to law-breaking water companies”.

A spokesperson for Water UK, the organisation that represents water companies, said that numbers of the most serious pollution incidents had fallen over the last decade, and stressed that the industry plans to invest £100bn into its network that will come into action once it receives approval from the regulator, Ofwat.